1. Field of the Invention
The present invention generally relates to packages for high-frequency devices, and, more particularly, to a package for a high frequency device that can perform impedance matching around each terminal in a package for a photo detector device such as PIN-PD (PIN photodiode) used for optical communication.
In recent years, as the transmission rates in optical communication have been becoming higher, there has been an increasing demand for packages that perform accurate characteristic impedance matching and accordingly have excellent transfer characteristics and reflection characteristics at high frequency and over a wide area.
Conventionally, a package for a high-frequency device normally uses ceramics as an insulator, because matching the characteristic impedance is relatively easy with ceramics. Since a metal wiring pattern is normally formed on the ceramics by metal plating, the characteristic impedance of signal lines formed by a Au plating film that is much thinner than the insulator can be easily calculated in a so-called parallel-plate-type arrangement.
However, a package using ceramics as an insulator is costly, and, therefore, there has been a demand for inexpensive packages using glass as an insulator. Still, the characteristic impedance needs to be accurately matched in packages using a glass insulator, as in packages using a ceramic insulator.
2. Description of the Related Art
FIG. 1 is a schematic perspective view of a package for a high-frequency device provided with glass insulators. In this figure, a metallic terminal 34 is fixed to a side wall of a metallic casing 31 by a glass piece 35, and extends along the side wall of the metallic casing 31 in a dual-in-line arrangement.
Although only one metallic terminal 34 is shown in this figure, a plurality of pairs of metallic terminals are actually fixed to the metallic casing 31. An optical fiber attachment portion 33 through which optical fibers for inputting and outputting light are attached to the metallic casing 31 is also formed on the front wall of the metallic casing 31. The metallic casing 31 is placed and fixed onto a stem 32 for mounting an optical device and peripheral circuits.
However, the above package having glass insulators is not provided with a mechanism for characteristic impedance matching, resulting in poor transfer characteristics and reflection characteristics at high frequency. More specifically, since each of the terminals that function as signal lines is much thicker than an Au plating film, the characteristic impedance can be calculated in the parallel-plate-type arrangement only if the gap between the casing and each terminal is very narrow, for instance, 0.1 mm. However, it is difficult to narrow the gap between the casing and each terminal to 0.1 mm, and matching the characteristic impedance is accordingly difficult. Furthermore, if the gap between the casing and each terminal is narrowed to 0.1 mm, there will be a strong possibility of short-circuiting due to contact between them.
A general object of the present invention is to provide packages for high-frequency devices, in which the above disadvantages are eliminated.
A more specific object of the present invention is to provide a package for a high-frequency device that performs accurate characteristic impedance matching while maintaining the gap between the casing and each terminal wide enough to avoid contact.
The above objects of the present invention are achieved by a package for a high-frequency device, comprising:
a plurality of metallic terminals that are insulated from a conductive casing by glass and are hermetically fixed to the conductive casing, each of the plurality of metallic terminals extending in parallel with a side wall of the conductive casing and being separated from the side wall; and
conductive protruding portions that are formed on either side of each metallic terminal and extend in the longitudinal direction of each metallic terminal, each pair of conductive protruding portions serving to match characteristic impedance.
With the conductive protruding portions that are formed on either side of each metallic terminal and extend in parallel with each metallic terminal, the gap between the conductive casing and each metallic terminal can be made wider than in a conventional package. For instance, the gap between the conductive casing and each metallic terminal can be as wide as 0.5 mm, and the possibility of contact between the conductive casing and each metallic casing can be reduced. Thus, the characteristic impedance can be accurately matched.
The above and other objects and features of the present invention will become more apparent from the following description taken in conjunction with the accompanying drawings.